1. Field of the Invention
The present invention relates to an apparatus for controlling the amount of slip of a lock-up clutch disposed between an engine and an automatic transmission of a motor vehicle.
2. Discussion of the Related Art
Some motor vehicles are equipped with a torque converter or fluid coupling which is disposed between an engine and an automatic transmission and which is provided with a lock-up clutch for direct connection of the engine and the automatic transmission. In this type of motor vehicle, the amount of slip of the lock-up clutch is controlled to an optimum value by suitable slip control means during deceleration of the vehicle, for the purpose of controlling the engine in a so-called "fuel cut" mode for a prolonged period of time to improve the fuel economy. For example, the engine is controlled in the fuel cut mode when the vehicle is coasting with the accelerator pedal placed in its non-operated position. In the fuel cut mode, a fuel supply to the engine is cut as long as the engine speed is higher than a predetermined fuel-cut speed, and the lock-up clutch is partially engaged with the slip amount being optimally controlled by a slip control device as disclosed in JP-A-1-279157.
This slip control device disclosed in the publication identified above is adapted to control a slip control hydraulic pressure to be applied to the lock-up clutch to regulate an engaging force of the lock-up clutch. When a change of the operating state of the vehicle which will cause subsequent deceleration of the vehicle is detected, the slip control device increases the slip control pressure to a relatively high level that produces a larger engagement force of the lock-up clutch during deceleration of the vehicle, than in a steady or stable running state of the vehicle at a substantially constant speed. The slip control pressure is held at that relatively high level for a given period of time to permit initiation of partial engagement of the lock-up clutch before the amount of slip of the lock-up clutch is controlled while the engine is controlled in the fuel cut mode.
When the operating state of the vehicle is changed leading to deceleration of the vehicle, for instance, when the accelerator pedal is released, the automatic transmission may be shifted from one position to another. Since the hydraulic pressure is being applied to the lock-up clutch for partial engagement thereof by the slip control device, the automatic transmission tends to suffer from a relatively large shifting shock, which is undesirable for smooth, comfortable running of the vehicle during coasting or deceleration of the vehicle.